1. Field of the Invention
The invention relates to a semiconductor device and a manufacturing method thereof.
2. Description of Related Art
In a semiconductor device in which an IGBT is formed, a buffer layer is formed between a drift layer and a collector layer to ensure voltage-resistance and the like. As described in Japanese Patent Application Publication No. 2010-141136 (JP 2010-141136 A), a buffer layer is typically formed by implanting impurity ions of a first conductivity type in part of a region that will become a drift layer of the first conductivity type of a semiconductor wafer.
When a buffer layer is formed by implanting impurity ions in the semiconductor wafer, if there is a location where there is a scratch on a surface of the semiconductor wafer on the side where the impurity ions are implanted, the buffer layer will become thin at this location. If the thickness of the buffer layer becomes thin, voltage resistance of the semiconductor device will tend to decrease. One method to increase the thickness of the buffer layer is to perform an annealing process for an extended period of time at a high temperature after the ion implantation. However, if a semiconductor substrate is kept at a high temperature for an extended period of time, a device structure already formed in an earlier process may become polluted or the like.